Gas-drying systems are used to remove moisture from gases, such as air, methane, and carbon dioxide. Moisture removal is typically achieved either by passing the moist gas through a desiccant or by flowing it over a heat exchanger surface that is maintained at a lower temperature than the flowed gas. Heat exchanger type gas dryers are typically much more compact than desiccant types gas dryers, and primarily for that reason, are more prevalent in the industry. Moreover, heat exchanger type gas dryers utilizing stacked plates provide yet further reductions in the size of the heat exchanger.
Typically, gas dryer heat exchangers utilize either a low temperature refrigerant, produced by a mechanical refrigeration cycle, or a low temperature fluid, such as water, to provide the cooling. The hot, moist gas is cooled to its dew point, which is the temperature at which vapor from the gas will condense on the heat exchanger surfaces, forming a liquid on the heat exchanger surfaces. At this point, the gas is in a thermodynamically saturated state. As the gas is further cooled, additional vapor is removed, although the gas remains saturated. Thus, the gas exiting the heat exchanger contains an amount of vapor, but has a reduced mass ratio of water-to-gas.
Saturated gas, whether at elevated or reduced temperature, is typically unsuitable for many industrial applications. Therefore, many industrial application systems incorporate a secondary heat exchanger to re-heat the existing reduced temperature, moist gas above its dew point and thereby provide a non-saturated gas to the industrial application system with significantly reduced moisture content. In many cases, the heated, moist gas flowing to the reduced temperature heat exchanger is used as the heat source for the re-heat process. For this application, the re-heat exchanger is known as a recuperator because thermal energy is removed from a gas stream entering the reduced temperature heat exchanger, the thermal energy being simultaneously recovered by a gas stream exiting the reduced temperature heat exchanger. Thus the recuperator increases the system efficiency by reducing the amount of cooling required to be accomplished in the reduced temperature heat exchanger. Manufacturers of brazed-plate type heat exchangers generally combine the reduced temperature exchanger and recuperator into a single assembly. The integral assembly is typically referred to as either an “integrated gas dryer” or a “recuperated gas dryer.”
A considerable amount of moisture is condensed from the gas while it flows through the reduced temperature exchanger. This moisture must be captured and removed prior to the gas entering the recuperator to prevent it from re-evaporating and thereby increasing the moisture level in the gas as the gas is heated. This moisture removal is accomplished through the use of a device known as a moisture separator.
Previous methods to accomplish this gas-drying include:                two separate heat exchangers and an external moisture separator;        one integral heat exchanger with an external moisture separator; and        one integral heat exchanger with an integral, external moisture separator.        
A basic flowpath of a gas through a heat exchanger/separator system is described for the two separate heat exchangers (recuperator and refrigerated dryer) with an external moisture separator design as shown in FIG. 1:                1) Heated, moist gas enters the recuperator at Point 1.        2) The heated gas flows through one portion of the recuperator (from Point 1 to Point 2) and is pre-cooled by gas flowing through another portion of the recuperator (from Point 7 to Point 8). The gas exits the recuperator at Point 2.        3) The gas then flows through external piping and enters the refrigerated dryer (reduced temperature heat exchanger) at Point 3.        4) The gas flows through the refrigerated dryer (from Point 3 to Point 4) wherein the gas is cooled to its dew point by a cooling fluid flowing through the refrigerated dryer between an inlet and an outlet.        5) The gas then exits the Refrigerated Dryer at Point 4 as a combination of cool, moist saturated gas and liquid water and flows through a conduit to an inlet of an external separator at Point 5.        6) The gas and liquid flow from Point 5 to Point 6 through the separator, which captures and separates the condensed liquid from the gas.        7) The gas then exits the external separator at Point 6 and flows through another conduit to the cold side of the recuperator at Point 7.        8) The gas then flows through the cold side of the recuperator from Point 7 to Point 8 wherein the gas is heated to a temperature greater than the dew point by the incoming heated gas (flowing from Point 1 to Point 2).        9) The gas exits the recuperator at Point 8 as a dried gas.        
An improvement to this method is to combine the two separate heat exchangers (recuperator and refrigerated dryer) into one integral heat exchanger with an external moisture separator. This combined heat exchanger reduces the manufacturing cost by eliminating one run of conduit between the heat exchangers, provides an increasingly compact heat exchanger construction, and increases system efficiency by eliminating the thermal energy loss formerly associated with transporting the gas through the conduit between the heat exchangers. A system schematic is shown in FIG. 2 and functions substantially similar as previously described for FIG. 1.
A further improvement to the previously described systems incorporates an integral, external moisture separator secured to one face of an integrated gas dryer assembly, i.e., recuperator and refrigerated dryer. As shown in FIG. 3, the separator section is secured to one face of the refrigerated dryer section. By making the separator section integral with either the recuperator or the refrigerated dryer section, the conduit is eliminated in steps 5) and 7) as previously discussed with regard to FIG. 1.
However, there are several significant drawbacks associated with this integral construction. First, to create the entire integrated unit, specially configured stamped plates are required for the heat exchanger sections. That is, single plates must each extend to encompass both the recuperator and refrigerated dryer sections, which plates are sized differently than the plates that comprise the separator section. Second, the design construction requires several differently sized stamped plates to cover a typical performance range of industrial interest. Third, the “footprint” size of the integral construction cannot be adjusted to optimize the customer's system package size and cost because the recuperator and refrigerated dryer sections must have the same number of plates, respectively. Fourth, a further disadvantage resulting from the recuperator and refrigerated dryer sections having the same number of plates is that the refrigerated dryer section cannot be changed or independently adjusted, which greatly restricts the range of application and performance of the device. Fifth, the integral construction design has significantly lowered pressure-bearing capabilities because the separator section is attached, and therefore only supported on one side by the recuperator or refrigerated dryer section. Sixth, the integral construction, as well as the construction as shown in FIG. 2, both suffer from a lower thermal efficiency caused by the recuperator section being directly coupled to the refrigerated dryer section. Due to this direct contact, thermal energy is transferred by virtue of conduction directly through the heat exchanger plates from one section to the next. In other words, since the recuperator section operates at a higher average temperature than the refrigerated dryer section, heat is transferred from the recuperator to the refrigerated dryer section, which increases the thermal load required from the refrigeration system, and reduces the re-heat temperature of the dry gas that exits the recuperator.
What is needed is an integral gas dryer construction that does not have the drawbacks as discussed above.